Auxin signal transduction.

The plant hormone auxin (indole-3-acetic acid, IAA) controls growth and developmental responses throughout the life of a plant. A combination of molecular, genetic and biochemical approaches has identified several key components involved in auxin signal transduction. Rapid auxin responses in the nucleus include transcriptional activation of auxin-regulated genes and degradation of transcriptional repressor proteins. The nuclear auxin receptor is an integral component of the protein degradation machinery. Although auxin signalling in the nucleus appears to be short and simple, recent studies indicate that there is a high degree of diversity and complexity, largely due to the existence of multigene families for each of the major molecular components. Current studies are attempting to identify interacting partners among these families, and to define the molecular mechanisms involved in the interactions. Future goals are to determine the levels of regulation of the key components of the transcriptional complex, to identify higher-order complexes and to integrate this pathway with other auxin signal transduction pathways, such as the pathway that is activated by auxin binding to a different receptor at the outer surface of the plasma membrane. In this case, auxin binding triggers a signal cascade that affects a number of rapid cytoplasmic responses. Details of this pathway are currently under investigation.

Inference of the Arabidopsis lateral root gene regulatory network suggests a bifurcation mechanism that defines primordia flanking and central zones.

A large number of genes involved in lateral root (LR) organogenesis have been identified over the last decade using forward and reverse genetic approaches in Arabidopsis thaliana. Nevertheless, how these genes interact to form a LR regulatory network largely remains to be elucidated. In this study, we developed a time-delay correlation algorithm (TDCor) to infer the gene regulatory network (GRN) controlling LR primordium initiation and patterning in Arabidopsis from a time-series transcriptomic data set. The predicted network topology links the very early-activated genes involved in LR initiation to later expressed cell identity markers through a multistep genetic cascade exhibiting both positive and negative feedback loops. The predictions were tested for the key transcriptional regulator AUXIN RESPONSE FACTOR7 node, and over 70% of its targets were validated experimentally. Intriguingly, the predicted GRN revealed a mutual inhibition between the ARF7 and ARF5 modules that would control an early bifurcation between two cell fates. Analyses of the expression pattern of ARF7 and ARF5 targets suggest that this patterning mechanism controls flanking and central zone specification in Arabidopsis LR primordia.

Molecular basis for AUXIN RESPONSE FACTOR protein interaction and the control of auxin response repression.

In plants, the AUXIN RESPONSE FACTOR (ARF) transcription factor family regulates gene expression in response to auxin. In the absence of auxin, ARF transcription factors are repressed by interaction with AUXIN/INDOLE 3-ACETIC ACID (Aux/IAA) proteins. Although the C termini of ARF and Aux/IAA proteins facilitate their homo- and heterooligomerization, the molecular basis for this interaction remained undefined. The crystal structure of the C-terminal interaction domain of Arabidopsis ARF7 reveals a Phox and Bem1p (PB1) domain that provides both positive and negative electrostatic interfaces for directional protein interaction. Mutation of interface residues in the ARF7 PB1 domain yields monomeric protein and abolishes interaction with both itself and IAA17. Expression of a stabilized Aux/IAA protein (i.e., IAA16) bearing PB1 mutations in Arabidopsis suggests a multimerization requirement for ARF protein repression, leading to a refined auxin-signaling model.

Structural basis for oligomerization of auxin transcriptional regulators.

The plant hormone auxin is a key morphogenetic regulator acting from embryogenesis onwards. Transcriptional events in response to auxin are mediated by the auxin response factor (ARF) transcription factors and the Aux/IAA (IAA) transcriptional repressors. At low auxin concentrations, IAA repressors associate with ARF proteins and recruit corepressors that prevent auxin-induced gene expression. At higher auxin concentrations, IAAs are degraded and ARFs become free to regulate auxin-responsive genes. The interaction between ARFs and IAAs is thus central to auxin signalling and occurs through the highly conserved domain III/IV present in both types of proteins. Here, we report the crystal structure of ARF5 domain III/IV and reveal the molecular determinants of ARF-IAA interactions. We further provide evidence that ARFs have the potential to oligomerize, a property that could be important for gene regulation in response to auxin.

ARF-Aux/IAA interactions through domain III/IV are not strictly required for auxin-responsive gene expression.

Auxin response factors (ARFs), together with auxin/indole acetic acid proteins (Aux/IAAs), are transcription factors that play key roles in regulating auxin-responsive transcription in plants. Current models for auxin signaling predict that auxin response is dependent on ARF-Aux/IAA interactions mediated by the related protein-protein interaction domain (i.e., referred to as the CTD) found in the ARF and Aux/IAA C-terminal regions. When auxin concentrations in a cell are low, ARF activators residing on the promoters of auxin response genes are thought to be inactive because of the association with dominant Aux/IAA repressors. When auxin concentrations are elevated, the Aux/IAA repressors are recruited to auxin receptors and degraded via the ubiquitin-proteasome pathway. Destruction of the Aux/IAA repressors allows the ARF activators to function in derepressing/activating auxin response genes. While this auxin signaling pathway is simple and attractive, it is unclear whether auxin-regulated gene expression is solely dependent on ARF-Aux/IAA interactions. Here we show that auxin can affect the expression of auxin response genes in a manner that is independent of the ARF activator CTD.

Getting a grasp on domain III/IV responsible for Auxin Response Factor-IAA protein interactions.

Auxin Response Factors (ARFs) and Indole Acetic Acid (IAA) proteins contain a similar carboxyl-terminal domain (domain III/IV) that facilitates interactions among these transcription factors as well as other proteins. The specificity of these interactions is controversial, and the mechanisms involved in these interactions have not been investigated. Here, we review some of the controversies about the specificities and requirements for ARF and IAA interactions and discuss some of the technical problems that might contribute to differences reported for these interactions. We make some preliminary conclusions that ARF activator-IAA, ARF activator-ARF activator, and ARF repressor-ARF repressor interactions are favored over ARF repressor-IAA and ARF repressor-ARF activator interactions, and we suggest that IAA-IAA interactions are largely indiscriminant. Based upon the predicted secondary structure of domain III/IV, we introduce a model for how ARF and IAA proteins might interact with one another through a ubiquitin-like ß-grasp fold.

Do some IAA proteins have two repression domains?

The plant hormone auxin regulates the transcription of specific genes through the interplay of Auxin Response Factors (ARFs) and Aux/IAA (IAA) repressors. We have recently shown that stabilized IAA repressors with identical amino acid substitutions in their conserved repression domains (i.e., domain I) confer either "low auxin" or "high auxin" phenotypes when the IAA proteins are constitutively expressed in transformed Arabidopsis plants. We have suggested that when domain I loses its capacity to repress, "high auxin" phenotypes generally result, but a subset of IAA proteins (e.g., IAA17) appear to contain a second repression domain resulting in the maintenance of "low auxin" phenotypes. Here we provide evidence for a second repression domain that lies between domains I and II in IAA7, an IAA repressor within the same clade as IAA17.

Identical amino acid substitutions in the repression domain of auxin/indole-3-acetic acid proteins have contrasting effects on auxin signaling.

Auxin/indole-3-acetic acid (Aux/IAA) proteins function as repressors of auxin response gene expression when auxin concentrations in a cell are low. At elevated auxin concentrations, these repressors are destroyed via the ubiquitin-proteasome pathway, resulting in derepression/activation of auxin response genes. Most Aux/IAA repressors contain four conserved domains, with one of these being an active, portable repression domain (domain I) and a second being an auxin-dependent instability domain (domain II). Here, we have analyzed the effects of amino acid substitutions in the repression domain of selected Aux/IAA proteins. We show that stabilized versions of Aux/IAA proteins with amino acid substitutions in domain I display contrasting phenotypes when expressed in transformed Arabidopsis (Arabidopsis thaliana) plants. An alanine-for-leucine substitution in the LxLxL (where L is leucine and x is another amino acid) repression domain of IAA3, IAA6, or IAA19 confers enhanced auxin response gene expression and "high-auxin" phenotypes when expressed from the 35S or IAA19 promoter (as tested with IAA19) in transformed Arabidopsis plants. In marked contrast, a single alanine-for-leucine substitution in domain I of IAA12 or IAA17 confers repression of auxin response genes and "low-auxin" phenotypes. These results point to intrinsic differences in the repression domain(s) of IAA proteins and suggest that some IAA proteins have stronger or more complex repression domains than others.

Constitutive repression and activation of auxin signaling in Arabidopsis.

Aux/IAA proteins are proposed to be transcriptional repressors that play a crucial role in auxin signaling by interacting with auxin response factors and repressing early/primary auxin response gene expression. In assays with transfected protoplasts, this repression was previously shown to occur when auxin concentrations in a cell are low, and derepression/activation was observed when auxin concentrations are elevated. Here we show that a stabilized version of the Arabidopsis (Arabidopsis thaliana) IAA17 repressor, when expressed constitutively or in a specific cell type in Arabidopsis plants, confers phenotypes similar to plants with decreased auxin levels. In contrast, a stabilized version of IAA17 that was converted to a transcriptional activator confers phenotypes similar to plants with increased auxin levels, when expressed under the same conditions in Arabidopsis plants. Free auxin levels were unchanged compared to control (DR5:beta-glucuronidase), however, in the seedlings expressing the IAA17 repressor and activator. These results together with our previous results carried out in transfected protoplasts suggest that the hormone auxin can be bypassed to regulate auxin signaling in a cell-autonomous manner in plants.

Subunit compositions of the RNA-silencing enzymes Pol IV and Pol V reveal their origins as specialized forms of RNA polymerase II.

In addition to RNA polymerases I, II, and III, the essential RNA polymerases present in all eukaryotes, plants have two additional nuclear RNA polymerases, abbreviated as Pol IV and Pol V, that play nonredundant roles in siRNA-directed DNA methylation and gene silencing. We show that Arabidopsis Pol IV and Pol V are composed of subunits that are paralogous or identical to the 12 subunits of Pol II. Four subunits of Pol IV are distinct from their Pol II paralogs, six subunits of Pol V are distinct from their Pol II paralogs, and four subunits differ between Pol IV and Pol V. Importantly, the subunit differences occur in key positions relative to the template entry and RNA exit paths. Our findings support the hypothesis that Pol IV and Pol V are Pol II-like enzymes that evolved specialized roles in the production of noncoding transcripts for RNA silencing and genome defense.

Auxin response factors.

Auxin signaling is key to many plant growth and developmental processes from embryogenesis to senescence. Most, if not all, of these processes are initiated and/or mediated through auxin-regulated gene expression. Two types of transcription factor families are required for controlling expression of auxin response genes. One of these, the auxin response factor (ARF) family, functions by binding to auxin response elements (AuxREs) on promoters of auxin response genes, activating or repressing the auxin response genes, and recruiting a second family of transcription factors, the Aux/IAA repressors, that confer an auxin response to the genes. Recent advances have provided information on regulation of ARF gene expression, ARF roles in growth and developmental processes, and target genes regulated by ARFs.

Transfection assays with protoplasts containing integrated reporter genes.

Transient expression assays with protoplasts that utilize stably integrated reporter genes along with transfected effector genes provide several advantages over assays in which both the reporter gene and effector gene(s) are transfected into protoplasts. A protocol for carrying out transient expression assays with Arabidopsis leaf mesophyll protoplasts containing single-copy integrated reporter genes is described.

AUXIN RESPONSE FACTOR1 and AUXIN RESPONSE FACTOR2 regulate senescence and floral organ abscission in Arabidopsis thaliana.

In plants, both endogenous mechanisms and environmental signals regulate developmental transitions such as seed germination, induction of flowering, leaf senescence and shedding of senescent organs. Auxin response factors (ARFs) are transcription factors that mediate responses to the plant hormone auxin. We have examined Arabidopsis lines carrying T-DNA insertions in AUXIN RESPONSE FACTOR1 (ARF1) and ARF2 genes. We found that ARF2 promotes transitions between multiple stages of Arabidopsis development. arf2 mutant plants exhibited delays in several processes related to plant aging, including initiation of flowering, rosette leaf senescence, floral organ abscission and silique ripening. ARF2 expression was induced in senescing leaves. ARF2 regulated leaf senescence and floral organ abscission independently of the ethylene and cytokinin response pathways. arf1 mutations enhanced many arf2 phenotypes, indicating that ARF1 acts in a partially redundant manner with ARF2. However, unlike arf2 mutations, an arf1 mutation increased transcription of Aux/IAA genes in Arabidopsis flowers, supporting previous biochemical studies that indicated that ARF1 is a transcriptional repressor. Two other ARF genes, NPH4/ARF7 and ARF19, were also induced by senescence, and mutations in these genes enhanced arf2 phenotypes. NPH4/ARF7 and ARF19 function as transcriptional activators, suggesting that auxin may control senescence in part by activating gene expression.

Auxin response factors ARF6 and ARF8 promote jasmonic acid production and flower maturation.

Pollination in flowering plants requires that anthers release pollen when the gynoecium is competent to support fertilization. We show that in Arabidopsis thaliana, two paralogous auxin response transcription factors, ARF6 and ARF8, regulate both stamen and gynoecium maturation. arf6 arf8 double-null mutant flowers arrested as infertile closed buds with short petals, short stamen filaments, undehisced anthers that did not release pollen and immature gynoecia. Numerous developmentally regulated genes failed to be induced. ARF6 and ARF8 thus coordinate the transition from immature to mature fertile flowers. Jasmonic acid (JA) measurements and JA feeding experiments showed that decreased jasmonate production caused the block in pollen release, but not the gynoecium arrest. The double mutant had altered auxin responsive gene expression. However, whole flower auxin levels did not change during flower maturation, suggesting that auxin might regulate flower maturation only under specific environmental conditions, or in localized organs or tissues of flowers. arf6 and arf8 single mutants and sesquimutants (homozygous for one mutation and heterozygous for the other) had delayed stamen development and decreased fecundity, indicating that ARF6 and ARF8 gene dosage affects timing of flower maturation quantitatively.

AUXIN RESPONSE FACTOR7 restores the expression of auxin-responsive genes in mutant Arabidopsis leaf mesophyll protoplasts.

AUXIN RESPONSE FACTOR7 (ARF7) is one of five ARF transcriptional activators in Arabidopsis thaliana that is proposed to regulate auxin-responsive expression of genes containing TGTCTC auxin response elements in their promoters. An Arabidopsis mutant (nonphototropic hypocotyl4-1 [nph4-1]) that is a null for ARF7 showed strongly reduced expression of integrated auxin-responsive reporter genes and natural genes that were monitored in Arabidopsis leaf mesophyll protoplasts. Expression of the reporter and natural genes was restored in an auxin-dependent manner when protoplasts were transfected with a 35S:ARF7 effector gene, encoding a full-length ARF7 protein. Transfection of effector genes encoding other ARF activators restored auxin-responsive gene expression to varying degrees, but less than that observed with the ARF7 effector gene. Arabidopsis lines that were null for ARF6, ARF8, or ARF19 were not defective in expression of the reporter and natural auxin response genes assayed in mesophyll protoplasts, suggesting that ARF7 plays a major role in regulating expression of a subset of auxin response genes in leaf mesophyll cells. Auxin-responsive gene expression was induced in wild-type protoplasts and restored in nph4-1 protoplasts only with auxin and not with other hormones, including brassinolide. In the presence of auxin, however, brassinolide modestly enhanced auxin-responsive gene expression.

NPH4/ARF7 and ARF19 promote leaf expansion and auxin-induced lateral root formation.

Auxin response factors (ARFs) bind auxin response promoter elements and mediate transcriptional responses to auxin. Five of the 22 ARF genes in Arabidopsis thaliana encode ARFs with glutamine-rich middle domains. Four of these can activate transcription and have been ascribed developmental functions. We show that ARF19, the fifth Q-rich ARF, also activates transcription. Mutations in ARF19 have little effect on their own, but in combination with mutations in NPH4/ARF7, encoding the most closely related ARF, they cause several phenotypes including a drastic decrease in lateral and adventitious root formation and a decrease in leaf cell expansion. These results indicate that auxin induces lateral roots and leaf expansion by activating NPH4/ARF7 and ARF19. Auxin induces the ARF19 gene, and NPH4/ARF7 and ARF19 together are required for expression of one of the arf19 mutant alleles, suggesting that a positive feedback loop regulates leaf expansion and/or lateral root induction.

Isolation of cucumber CsARF cDNAs and expression of the corresponding mRNAs during gravity-regulated morphogenesis of cucumber seedlings.

Cucumber seedlings show positive gravitropism and bend in the transition zone between the hypocotyl and the root. The peg, a specialized protuberance, develops on the concave side of the bending transition zone. Auxin and the mRNA of an auxin-inducible gene (CsIAA1) isolated from cucumber are differentially accumulated across the transition zone during the gravity-regulated peg formation. In this study, five cDNAs of Auxin Response Factors (ARFs) from cucumber were isolated and their mRNA accumulation was compared with that of CsIAA1. The tissue specificity of CsARF2 mRNA accumulation was similar to that of CsIAA1. Because the structural character of CsARF2 predicts that it is a transcriptional activator, CsARF2 may be involved in the activation of CsIAA1 transcription, which plays a role in gravity-regulated peg formation. Neither gravity nor auxin affected mRNA accumulation of five CsARFs including CsARF2, suggesting that CsARF2 may be regulated at a post-transcriptional level to induce the asymmetric expression of the CsIAA1 gene in response to gravistimulation and auxin in cucumber seedlings.

Overlapping and non-redundant functions of the Arabidopsis auxin response factors MONOPTEROS and NONPHOTOTROPIC HYPOCOTYL 4.

Transcription factors of the auxin response factor (ARF) family have been implicated in auxin-dependent gene regulation, but little is known about the functions of individual ARFs in plants. Here, interaction assays, expression studies and combinations of multiple loss- and gain-of-function mutants were used to assess the roles of two ARFs, NONPHOTOTROPIC HYPOCOTYL 4 (NPH4/ARF7) and MONOPTEROS (MP/ARF5), in Arabidopsis development. Both MP and NPH4 interact strongly and selectively with themselves and with each other, and are expressed in vastly overlapping domains. We show that the regulatory properties of both genes are far more related than suggested by their single mutant phenotypes. NPH4 and MP are capable of controlling both axis formation in the embryo and auxin-dependent cell expansion. Interaction of MP and NPH4 in Arabidopsis plants is indicated by their joint requirement in a number of auxin responses and by synergistic effects associated with the co-overexpression of both genes. Finally, we demonstrate antagonistic interaction between ARF and Aux/IAA gene functions in Arabidopsis development. Overexpression of MP suppresses numerous defects associated with a gain-of-function mutation in BODENLOS (BDL)/IAA12. Together these results provide evidence for the biological relevance of ARF-ARF and ARF-Aux/IAA interaction in Arabidopsis plants and demonstrate that an individual ARF can act in both invariantly programmed pattern formation as well as in conditional responses to external signals.

Aux/IAA proteins contain a potent transcriptional repression domain.

Aux/IAA proteins are short-lived nuclear proteins that repress expression of primary/early auxin response genes in protoplast transfection assays. Repression is thought to result from Aux/IAA proteins dimerizing with auxin response factor (ARF) transcriptional activators that reside on auxin-responsive promoter elements, referred to as AuxREs. Most Aux/IAA proteins contain four conserved domains, designated domains I, II, III, and IV. Domain II and domains III and IV play roles in protein stability and dimerization, respectively. A clear function for domain I had not been established. Results reported here indicate that domain I in Aux/IAA proteins is an active repression domain that is transferable and dominant over activation domains. An LxLxL motif within domain I is important for conferring repression. The dominance of Aux/IAA repression domains over activation domains in ARF transcriptional activators provides a plausible explanation for the repression of auxin response genes via ARF-Aux/IAA dimerization on auxin-responsive promoters.

The roles of auxin response factor domains in auxin-responsive transcription.

Auxin response factors (ARFs) are transcription factors that bind to TGTCTC auxin response elements in promoters of early auxin response genes. ARFs have a conserved N-terminal DNA binding domain (DBD) and in most cases a conserved C-terminal dimerization domain (CTD). The ARF CTD is related in amino acid sequence to motifs III and IV found in Aux/IAA proteins. Just C terminal to the DBD, ARFs contain a nonconserved region referred to as the middle region (MR), which has been proposed to function as a transcriptional repression or activation domain. Results with transfected protoplasts reported here show that ARFs with Q-rich MRs function as activators, whereas most, if not all other ARFs, function as repressors. ARF DBDs alone are sufficient to recruit ARFs to their DNA target sites, and auxin does not influence this recruitment. ARF MRs alone function as activation or repression domains when targeted to reporter genes via a yeast Gal4 DBD, and auxin does not influence the potency of activation or repression. ARF CTDs, along with a Q-rich MR, are required for an auxin response whether the MRs plus CTDs are recruited to a promoter by an ARF DBD or by a Gal4 DBD. The auxin response is mediated by the recruitment of Aux/IAA proteins to promoters that contain a DNA binding protein with a Q-rich MR and an attached CTD.